The present invention is in the field of electrical generators and, more particularly, electrical generators which operate under rapidly varying load conditions.
Generators used in aircraft or space vehicles are often operated under variable load conditions. Typically, these generators are provided with regulators that modulate generator field current based on a detected voltage at a point of regulation (a POR). This detected voltage is referred to as a POR voltage. In many applications of these variable load generators, a POR voltage must be maintained within narrowly defined limits. Failure to maintain a POR voltage at a desired level may result in damage to equipment to which the generator supplies power.
In many aircraft or spacecraft applications, electrical generators are subjected to widely varying and rapidly changing load conditions. When load is rapidly removed from a generator, it is difficult to prevent POR voltage from rising above a desired level. This is because rapid removal of load requires rapid reduction of current in the exciter winding to keep the POR voltage within the desired limit. However, the exciter winding current cannot be reduced to zero instantly due to the inductive nature of the winding. A residual amount of energy in an exciter winding of the generator continues circulating through a freewheeling diode for a short time and this energy contributes to a short-term rise in POR voltage. In the prior art, this residual energy has been discharged into an impedance circuit when POR voltage rises as a result of a rapid off-loading of the generator. One particularly, effective technique for discharging this residual energy is described in U.S. Pat. No. 6,628,104, issued to Yuan Yao et al. on Sep. 30, 2003.
But, even with availability of sophisticated discharging techniques for residual exciter energy, there still remains a problem in the maintenance of POR voltage during rapid load-off conditions. In the prior art, energy discharging systems have been activated or triggered in response to signals from voltage detectors. When a POR voltage rose above a predetermined level during load reduction, the discharge system would be triggered. But a triggering technique based on measuring POR voltage is inherently limited in the degree of precision with which POR voltage may be controlled. Such a technique requires that triggering not occur at a voltage that is at or below the predetermined POR voltage level. Triggering may only occur after POR voltage exceeds the predetermined POR voltage level.
After triggering is performed there is an inherent time delay before POR voltage is effectively reduced by discharging the residual energy. During this time delay POR voltage continues to rise. This presents a doubly problematic situation. First of all, as stated above, a triggering voltage threshold must be selected which is higher than the desired POR. Secondly, there must be consideration given to the fact that POR voltage will rise even higher during a time delay after triggering. In the prior art there has never been a generator control system that completely precludes an overvoltage condition from developing during a rapid load-off event.
Consequently, any equipment driven by the prior-art generators must be robust enough to tolerate a POR overvoltage without being damaged. This of course means that the driven equipment must be built with a certain factor of safety. This translates into undesirable increases in size and weight of the equipment. Excessive size and weight are properties that must be avoided in aircraft and spacecraft equipment.
As can be seen, it would be desirable to provide electrical generators in which precise control of POR voltage may be maintained during rapid load varying conditions. In particular, it would be desirable to provide a control system which precludes an overvoltage at the point of regulation during a rapid load-off event.